The long term goal of this project is to explore the potential of polyclonal antibody libraries as therapeutic and/or diagnostic tools in the treatment of diseases such as cancer, microbial infections, autoimmunity, and immunodeficiency, as well as in efforts to prevent transplant rejection and graft versus host disease. Polyclonal antibody libraries would combine the advantages of targeting multiple antigenic determinants (high avidity, low likelihood of antigen escape variants, and efficient mediation of effector functions) with the advantages of using monoclonal antibodies (unlimited supply of standardized reagents, and the availability of the genetic material for desired manipulations). The selection of polyclonal antibody libraries with desired specificities is made possible by the technology for generating Fab phage display libraries. To adapt this technology for the creation of specific libraries of intact IgG antibodies, the P.I.s laboratory has recently constructed bidirectional phage display and mammalian expression vectors. In these vectors, the heavy (H) and light (L) chain transcription units are in divergent orientations to facilitate the bulk transfer of physically linked pairs of variable (V) region genes (VL-VH) from the phage display vector to the mammalian expression vector, without loss of the selected VL-VH combinations. In the current grant application, The investigator proposes to continue to develop and optimize the technology for production of polyclonal antibody libraries, using as a model system the CD4+ subset of human T lymphocytes. The specific aims of the current application are: to generate polyclonal Fab phage display libraries (directed against human CD4+ T-cells) derived from two different species (mouse and chicken); to generate a Fab phage display sublibrary selected against reactivity with T-cell-depleted human peripheral blood mononuclear cells and a sublibrary additionally selected against reactivity with CD8+ T-cells; to express the selected polyclonal libraries as libraries of intact IgG antibodies with mouse or chicken V regions and mouse or functions in vitro; and to test the V region immunogenicity of the IgG libraries.